The disaster almost defies imagination: a crowded boat capsizes by night, casting hundreds of people into the Mediterranean.For years, countless migrants have tried to reach Europe on leaking rust-buckets provided by amoral traffickers. There was a terrible air of inevitability about the latest tragedy off the Italian island of Lampedusa. Last year, 140,000 migrants crossed from North Africa to Italy. The signs are that even more will make the passage in 2015; already, some 1,600 people have drowned in the attempt.Where do the migrants come from and why this sudden surge of arrivals? We know that events on the far side the world help to determine how many desperate people will embark on an epic journey which they hope will end in Europe.What is less well known is that the recent escalatio